Today is the first day of September 9th. The city where Xiaobian is located has begun to snow heavily and the weather is cold. So how many days do you know in total? Why do you say that September 9th is the coldest time of the year?
January 9-17
The old man often said 'count nine cold days, cold in September', the coldest day is coming. The coldness of this period can also be seen from the saying of "no action in 1929, walk on the ice in 3949". "39" is the third nine days after the winter solstice, about the end of January. It comes from our country's ninth tradition. To say that the number nine is from the beginning of the winter solstice every year, one 'nine' every nine days, until the eighty-one day of 'nine nine', the warm weather is over.
"39" is the coldest time of the year
There is no exact information about the origin of the custom. To be sure, at least in the southern and Northern Dynasties, the custom of counting nine had become popular. Liang Daizong's Jingchu chronicle of the age of Jin and Chu said: 'it is customary to use winter solstice days and ninety-one days for the cold. '
The data emperor selected four cities with a long history and complete meteorological data records, namely Beijing, Kaifeng, Nanjing and Xi'an, to represent North China, central China, East China and Northwest China, and used the temperature data of 60 years since they had meteorological records (1951) as the basis.
According to the data from 1951 to 2010, the PK results of average temperature and extreme minimum temperature in these four cities during the period of September 9th and September 4th are as follows:
In terms of the average temperature, except for Xi'an, the other three cities all support a cooling ratio of 39 to 49. What about the extreme minimum temperature?
From the point of view of extreme minimum temperature, the four cities represent North China, central China, East China, northwest and other regions, which are obviously in line with the saying of "three nineties are colder". It seems that the folk saying "cold in September" has been well confirmed.
Why is it the coldest day
We know that the earth absorbs the heat of solar radiation during the day and emits it at night. In summer, it absorbs more heat in the daytime and emits less heat in the evening. It can retain some heat every day. In winter, it absorbs less heat in the daytime and emits more heat in the evening. Not only can it not be retained every day, but also the heat retained in summer can be used up.
On the day of the winter solstice, the direct sunlight is in the southern hemisphere, while the northern hemisphere absorbs less heat during the day. Fortunately, there is still some stored heat. After the winter solstice, the heat stored in the northern hemisphere is less than that in a day, and the heat stored in the northern hemisphere is the lowest in a year, so it is the coldest.
That is to say, when the sun's heat absorbed by the ground is almost equal to the heat emitted from the ground, the temperature reaches the coldest level.